This post has been updated.

Science fiction movies frequently make it seem as though hostile extraterrestrials might be able to sneak through Earth’s atmosphere, emerge from the clouds without warning, and start wrecking shit. But is it really possible that spaceships could suddenly appear out of the vast blackness of space unnoticed by the myriad instruments on the ground? Depending on the aliens’ level of technological sophistication, one can argue that almost anything is possible. Humans have pointed a lot of sensors skyward, but it’s not clear they would really give us a proper heads up before our extraterrestrial guests arrive.

It sort of depends on exactly what we’re looking for. The search for extraterrestrial intelligence, better known as SETI, has been fruitless so far, but we keep investing and more and more money towards projects aimed at answering whether or not we’re alone in the universe. That investigation basically revolves around detecting various signals that point to the existence of technologically advanced extraterrestrials. And that also means the instruments we can use to observe space let us see farther into the universe than ever before. How far exactly? Douglas Vakoch tells Inverse that the answer has more to do with defining “see” than with arriving at a specific number.

A SETI researcher and the president of METI International, Vakoch divides the tools used to do SETI research into two groups: optical, and radio. Optical instruments, which look for lasers or other light patterns that suggest the presence of intelligent aliens, can make decent observations up to about 1,000 light-years away. “Beyond 1,000 light-years, however, it can get more difficult,” Vakoch says. Those instruments include the Boquete Optical SETI Observatory in Panama, and the Owl Observatory in Michigan.

Radio instruments, on the other hand, make a lot more sense for investigating signs of extraterrestrial life. For one, we can listen in on radio waves moving through space from about 10,000 light-years and beyond. Radio waves are also a pretty specific sign of a technologically advanced civilization while a star system may be expressing a strange laser-based signal in the distance, that might also just be an unexplained natural phenomenon (as we’re sort of learning when it comes to Tabbys Star.

Of course, as Vakoch emphasizes, our abilities to listen in on the universe dwindles as the distance increases. Aliens broadcasting themselves 10,000 light-years away will likely avoid detection.

But then again, SETI is a two-way street. When asked whether we currently have a maximum limit from which we can detect alien life, Seth Shostak, the director of the Center for SETI Research at the SETI Institute, says, “It depends on how much money the aliens have to invest in their own technology.” He’s referring to the fact that intelligent lifeforms might possess technologies that allow them to broadcast their existence to us from farther away and offset the limits of our own instruments.

“In theory, there’s no limit,” he adds.

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It’s important to remember that SETI investigations don’t just watch the universe in all direction in hopes we’ll stumble on something. Scientists focus our instruments on certain regions of space that are most promising — i.e. the exoplanets or star systems that possess the best chances for developing habitable environments.

“We start close to home,” says Vakoch, “and look at sun-like stars. As we move the investigation farther out, we start to hone in on stars or star regions that might be useful. For example, a current trend in the SETI community is the study of red dwarf stars, which were historically ignored. The beauty of red dwarfs is that they last a long time,” says Vakoch, sustaining planetary systems that have a much longer chance of developing habitable traits. Red dwarfs make up about 80 to 90 percent of the stars in our own neighborhood within the Milky Way galaxy.”

There’s another big factor to consider. SETI research focuses on finding aliens that want to be found — or at the very least, are not actively trying to hide from human detection. If a hostile fleet of extraterrestrial warships were stationed somewhere in the galaxy or was en route to the solar system, they could very well possess a number of highly advanced technologies that could shroud their presence. Space itself is incredibly big anyways — humans don’t have eyes and ears scanning all parts of the universe around us.

Aliens might be able to slip into orbit, but if they were traveling in anything truly large it might be quite difficult for them to do so without severely disrupting our orbital equipment or even taking out a geosynchronous satellite. Still, that’s not an impossible feat.

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“If they’re sufficiently intelligent,” says Vakoch, “there’s no reason to think they couldn’t be anywhere.”

Photos via District 9, Guardians of the Galaxy